Project Summary Due to their multiple roles in adaptive immunity, induction of robust CD4 T cells (CD4) is a desirable goal for any preventative vaccine. Since these cells are the primary targets of HIV infection, their antiviral role in HIV immunity has been relatively understudied. We have shown immune pressure by effector CD4 drive viral escape, but the role of these cells and the implications of CD4 immune escape in HIV pathogenesis is unclear. We linked amino acid changes (termed mutations or adaptations) to specific HLA-II alleles and defined adapted epitopes (AE), where single amino acid substitutions reflect CD4-mediated immune pressure. The complementary non- adapted epitope (NAE) shows no evidence of mutation when assessed at a population level. These adaptations can accrue in a viral population, and the extent of adaptation in a new host is dependent upon their respective HLA-II alleles. We previously confirmed that AE represent immune escape from CD4 T cells and found the magnitude and function of AE-specific CD4 T cell responses were diminished, but the impact of CD4-restricted epitope adaptations on CD4 T cell immunity is unknown. Our overall hypothesis is that CD4 T cells targeting- NAE have a distinct functional phenotype able to control HIV replication, which is compromised by viral adaptation to these responses. Since initiating ART immediately after infection preserves CD4 T cells, this application will address whether early ART promotes better viral control by allowing CD4 T cells to target NAE, responses known to be beneficial as compared to AE. We will test this hypothesis using a unique cohort of clade B acutely infected Peruvians where patients receive antiretroviral therapy (ART) either immediately or 6 months post infection. In specific aim 1, we will determine the biologic relevance of HIV adaptation to CD4 T cells. To do so, we will determine the ability of CD4 T cells to inhibit virus that is fully adapted or non-adapted to these responses. We will also assess the extent to which HLA-II adaptation in the transmitted virus influences clinical disease course in that individual. In specific aim 2, we will decipher the functional differences (globally and at single cell level) that define an AE or NAE specific CD4 T cell response and determine whether immediate ART optimizes CD4 T cells by targeting NAE encoded in the infecting virus. In specific aim 3, we will determine the impact of CD4 T cell adaptation on vaccine immunogenicity and efficacy. We will evaluate the function of CD4 T cells in vaccinees as it relates to NAE and AE responses. We will also determine whether adaptation of the vaccine insert influences rates of HIV infection. In summary, CD4 T cell mediated escape implies an in-vivo survival advantage of HIV adaptation and suggests CD4 T cells are indeed important to immune control of HIV. Elucidating the mechanisms of CD4 T cell immunity in HIV infection and the relevance of HIV escape to these responses will benefit the design of future HIV-1 preventative vaccine strategies.